1. Field of the Invention
This invention relates to the production of beta-alumina ceramic articles.
2. Prior Art
The family of materials known under the generic name of beta-alumina are examples of polycrystalline solid electrolytes of the kind, sometimes referred to as superionic conductors, in which the polycrystalline grains have a framework structure, which permits rapid ionic diffusion of alkali metal ions in channels through the framework structure. Beta-alumina ceramic is well known as a polycrystalline ceramic material comprising primarily aluminium oxide with a small proportion of sodium oxide. The amount of sodium oxide in practice may range from 5% up to 10% by weight. The material may also contain up to 5% by weight of dopants, such as magnesium oxide or lithium oxide or various combinations of such dopants. The characteristic crystal structure of beta-alumina makes it useful as a separator, for example, in sodium sulphur cells and other electrochemical devices requiring the passage of sodium ions. Conventionally, beta-alumina ceramic articles are formed by compressing a powder, which may be powdered beta-alumina ceramic or may be a powdered mixture of oxide materials corresponding to the required overall composition to form the beta-alumina ceramic, and sintering this compressed powdered article at a suitable sintering temperature, which generally lies in the range 1500.degree. to 1900.degree. C. Methods of manufacturing of beta-alumina ceramic to the quality and integrity of other engineering ceramics are now well known. Generally it is necessary to control the grain size, the eliminate open porosity and reduce the closed porosity to a level of less than 5% by volume and preferably to a level of less than 1% by volume. It is known to produce beta-alumina by sintering in a closed crucible, see for example U.S. Pat. Nos. 3,404,036; 3,468,719 and 3,475,225. However, more recently, zone sintering techniques have been developed in order to produce materials of higher density and better mechanical integrity and an improved durability in electrochemical energy conversion devices such as sodium sulphur cells. These techniques are described in a paper by I. Wynn Jones and L. J. Miles "Production of B--Al.sub.2 O.sub.3 Electrolyte" published in the Proceedings of the British Ceramic Society, No. 19, (March 1971) pp 161-178 and in U.S. Pat. Nos. 3,950,463; 3,903,225 and 3,869,019 and require the article to be sintered to be passed rapidly through a tubular furnace.
In the specification of British application No. 43183/74, I have described and claimed a method of producing beta-alumina ceramic articles by compressing powdered beta-alumina or a mixture of powdered materials which, on heating, produce beta-alumina, and sintering the compressed powder material to form an impervious ceramic material of at least 98% theoretical density, wherein the article, after sintering, is pressurised in a gaseous pressure medium at an elevated temperature below but within 500.degree. C. of the sintering temperature. In that method, the article, after sintering, is subjected to hot isostatic pressing. The present invention relates to further improvements in the use of hot isostatic pressing, after sintering, of articles of beta-alumina ceramic.
It is known to improve the density of carbide materials and oxide ceramic materials by sheathless hot isostatic pressing. British patent specification No. 1,300,864 describes a method of producing a sintered article from a powdered material, in which a body of the powdered material is first sintered at atmospheric pressure or preferably under a vacuum and at such a temperature that the powdered particles are bound together, after which the sintered body is isostatically pressed to a high density, for example in a furnace in which the furnace is contained within a pressure chamber, under the direct influence of a pressure medium such as argon, helium, nitrogen or hydrogen, without being encapsulated in a sheath. Unlike beta-alumina ceramic, which is essentially a single phase material, the carbide materials, are two-phase materials and they are hot isostatically pressed at a temperature slightly above the liquids of the binder phase so that material is transported in the liquid phase. Carbide cutting tools manufactured by this method have an improved wear resistance, and a reduced risk of edge breaking, because of the elimination of pores. Other items, such as wire drawing dies, are strengthened because of the elimination of flaws. Further details of the process for improving sintered carbide materials are described by Lardner and Bettle in their article commencing on page 540 of the December 1973 Issue of the Journal "Metals and Materials".
The above-mentioned British patent specification No. 1,300,864 refers to the improving of the density of oxide ceramic materials by hot isostatic pressing for the oxide ceramic UO.sub.2. However, the method described therein specified that the initial sintering takes place in a hydrogen gas atmosphere or in vacuum, and this leaves a vacuum or an atmosphere which is diffusible with respect to UO.sub.2 polycrystalline lattice in the residual porosity. Thus, when the structure is densified in the second stage high temperature pressurisation, high pressures are not generated in closed porosity because the gases in the pores are able to escape by diffusion through the ceramic body.
More details of the process for use with polycrystalline oxide ceramics are described in U.S. Pat. No. 3,562,371. An article "Gas Isostatic pressing without moulds" in Part 2 of volume 54 of the Ceramic Bulletin published in 1975 reviews the background and provides further details of hot isostatic pressing of oxide ceramics, emphasising the benefits of reduced porosity for optical transmission in alumina envelopes for the high pressure sodium lamp, and transparent (Pb La) (Zr,Ti)O.sub.3 (PLZT) ceramics for electro optical applications.
It may be seen, therefore, that the known art of forming of oxide ceramics extends to the concept of a combination of a defined sintering process with pressurisation at an elevated temperature.
In the description of the methods of prior art in the aforementioned documents there is no specification of the cooling process which follows the hot pressing stage, although in a description of a preferred embodiment in column 5 of the specification of U.S. Pat. No. 3,562,371 it is stated that "after 1 hour, the pressure is released and the sample is cooled to ambient temperature". There is no mention of the process variables of the cooling part of the cycle in the previously mentioned article "Gas isostatic pressing without moulds". It would appear, therefore, that the known art of hot gas isostatic pressing of oxide ceramics would lead one experienced in the art to release the pressure before the ceramic is cooled, after the pressurisation stage. Such a procedure would be expected to minimise the residual stresses, which would be expected to be present after the ceramic is subjected to deformation during the pressurisation process. Such stresses would be expected to reduce the mechanical integrity of a brittle oxide ceramic. It is believed that a polycrystalline ceramic in which the grains are anisotropic in their deformation characteristics, i.e., have different Youngs modulus in the different crystal axes, would contain damaging residual stresses if cooled under pressure after a hot deformation process.
It is noteworthy that there has been no application of mouldless isostatic pressing at elevated temperatures to the framework structured superionic conductors, and a logical reason for this could lie in a concern by those skilled in the art of manufacturing these materials about the thermodynamic stability of these materials at elevated temperatures and pressures. Further details are given in a report entitled "Development of Sodium Sulphur Batteries for Utility Application" EPRI EM-266 (Research Project 128-3) which was an annual report of the Electrochemistry Branch of the Research and Development Centre of the General Electric Company, Schenectady, N.Y. 12301, prepared for the Electric Power Research Institute, 3412 Hillview Avenue, Palo Alto, Calif. 94304. The decomposition of .beta. and .beta." variants of beta-alumina at relatively low pressures and at an elevated temperature is discussed on pages E55 and E56 of the report.
The durability of polycrystalline beta-alumina ceramic materials in sodium sulphur cells is known to be affected by the development of internal stresses in the solid electrolyte. One known mechanism for the development of such damaging internal stresses is the substitution of alternative ions, such as potassium, into the lattice in place of the sodium ions. In the article "Beta-alumina electrolytes" in Progress in Solid State Chemistry Vol. 7 (J. O. McCaldin, Howard Reiss, Editors, Pergamon, N.Y. 1972), J. T. Kummer describes on page 155 how ion exchanging of potassium into sodium beta-alumina ceramic by immersing said ceramic in molten sodium nitrate causes the polycrystalline ceramic to fracture. It is also known that ion exchange from potassium containing glasses, used to seal the beta-alumina membrane to other portions of the cell, can reduce the durability of the electrolyte in sodium sulphur cells. Potassium is a larger ion than sodium, and so when it is ion exchanged for sodium in beta-alumina the `C` axis expands by approximately 0.2A.degree.. There is little change in the `a` axis dimension. Thus in a polycrystalline lattice of randomly oriented grains such ion exchange causes the `c` axis of the polycrystals to be placed in compression and the `a` axes to be placed in tension